Polypropylene, which has excellent stretching characteristics, insulation property, and voltage resistance, has been widely used for a film for a film capacitor. With the demand for film capacitors increasing mainly in the fields of automobiles, home electrical appliances, and the like, in order to meet the demand for further size reduction, high reliability, and cost reduction, films used therefor have been demanded to have a further improved breakdown voltage. In general, when there is a difference of about 10% in BDV, it is regarded as a significant difference.
As a film for a film capacitor, for example, Patent Document 1 discloses a film comprising a composition mainly composed of a high stereoregularity polypropylene.
However, none of polypropylenes with improved stereoregularity has been able to provide a film having a sufficient breakdown voltage, and in particular, a high breakdown voltage in a harsh environment with a high temperature and a high frequency current, and thus a capacitor satisfying the market demand has not been provided. In recent years, with progress in practical realization of high efficiency mainly especially for power converters by, for example, increasing the frequency of switching elements or driving them at high temperatures, a demand for parts that can be applied for realization of high reliability, size reduction, and cost reduction has been increasing.
Patent Document 2 describes a polypropylene composition comprising polypropylene and an α crystal nucleating agent, which can be used for a capacitor film. However, since the proportion of <2,1> erythro regio defects measured using 13C-NMR is as high as 0.1% by mol for the polypropylene described in Patent Document 2, a film having high a BDV especially at a high temperature cannot be obtained using the polypropylene because of its low melting point, etc. In addition, it is impossible to provide a film having a high breakdown voltage even in a harsh environment with a high frequency current by using the polypropylene because it is difficult to produce a film having preferable thickness accuracy. Accordingly, a high quality capacitor film cannot be provided. Patent Document 2 merely discloses, as an object of the invention, the improvement of fluctuation between capacitor film samples (i parameters), and therefore, it fails to teach or suggest the improvement of BDV. Further, it is impossible to achieve excellent stretchability and uniformity in stretching with the polypropylene disclosed in Patent Document 2.
Patent Document 3 describes an original sheet or film having a high β crystal fraction, which can be used for a capacitor film. However, even with the use of the film, it is impossible to provide a film having excellent stretchability and a sufficient breakdown voltage because it has, for example, a narrow range of temperatures available for stretching, thereby making impossible to provide a high-quality capacitor film.